The challenge for future missions

The concept of the next generation of the gravity field missions is already under development.

One of the most recent studies presents a mission concept, the Earth System Mass Transport Mission (Square) (e².motion), which is based on two pairs of satellites orbiting the Earth in two different orbits with 90^◦ and 70^◦ inclination, using the laser interferometry as the primary technique for inter-satellite ranging (NGGM-D Teamet al., 2014).

A mission concept development is a very complex task which requires extensive expertise and cooperation of the scientific and industrial community and which needs to be solved iteratively.

The approach for the e².motion concept development is shown in Figure 7.4. At first, based on the needs of the geoscientific community, the science requirements were defined in terms of temporal and spatial resolution of the gravity field models. Then a mission baseline scenario, i.e. the fundamental observation techniques and orbital configuration, were selected. To fulfill the science and mission goals, requirements on the AOCS were defined and a concept for AOCS designed and tested. At the same time, the instrument concept, i.e. design and performance requirements on the laser ranging interferometer and the ultra-sensitive accelerometer, were developed and tested. Finally, the satellite observations were simulated and verified in an end-to-end simulator.

Figure 7.4:e².motion concept development approach (NGGM-D Teamet al., 2014)

The AOCS concept includes a design and performance study for attitude determination sensors and attitude actuators. The AOCS needs to fulfill science and mission requirements as well as requirements coming from the operation of the scientific instruments. In the e².motion study, the AOCS concept includes detailed design and testing only of a few selected components, namely of the thruster attitude control for drag compensation, laser beam steering control loop using DWS and SM, orbit and attitude control for initial acquisition of the laser inter-satellite ranging, and orbit control for maintenance of the inter-satellite formation. For attitude determination, star cameras, IMU, CESS and magnetometer were considered (NGGM-D Team et al., 2014). However, no further information and requirements on the design, measurement accuracy or data processing of these attitude determination sensors were provided.

The GRACE mission analysis results presented in the previous chapters indicate that the impact of the attitude determination on the satellite operation and on the scientific data processing must be taken into account already when developing a new satellite mission concept.

An improved concept for attitude determination and upgraded satellite payload and data processing algorithms need to be designed and implemented in order to fully exploit the

measurement accuracy of the scientific instruments and to optimally support the in-flight satellite operation. As the next generation of the gravity field missions is currently in the concept development phase, indeed, the experience from the previous satellite missions can and should be incorporated in the design study in order to find a best fitting concept for the attitude determination and control system. Possibly, this might also include further attitude sensor technology development.

For this purpose we have designed a basic approach for the determination of the requirements on the measurement accuracy of the attitude determination sensors, on the accuracy of the calibration parameters related to the attitude sensors and on the in-flight and on-ground data processing, see Figure 7.5. We focus here on the star cameras and on the fine pointing mode, in which the satellites are expected to operate most of the mission lifetime as in this mode all scientific observations needed for the gravity field recovery are collected. This approach, however, can be adjusted for any other operational mode and the relevant primary attitude determination sensor.

In the first step of the proposed approach, the role of the attitude determination for the mission operation and for the scientific data processing is identified based on the science and mission requirements, the instrument concept and the attitude control system. In case of the mission operation, the precise attitude determination is necessary for the maintenance of the inter-satellite pointing. The attitude data accuracy also influences the propellant consumption when using thrusters for attitude control. Also, for the GRACE Follow-On like missions, the data stream from the laser beam steering control loop will be used directly in the satellite’s attitude determination and control loop. Based on the identified role, the requirements on the accuracy of the in-flight attitude solution, which is used as input for the attitude control loop, are defined.

Analogously, the role of attitude determination for the scientific data processing is identified.

Here, the precise attitude data are required for the post-processing of the scientific observations, i.e. for correction of the pointing-induced errors from the ranging observations, for rotation of the accelerometer data into the inertial frame and for rotation of the GNSS phase center offset vector from the satellite-fixed reference frame into inertial frame. Considering the target accuracy of the scientific observations, the requirements on the accuracy of the final attitude solution, which is computed on-ground, are defined. Note that the listed role of the attitude determination for both the mission operation and the scientific data processing is only for illustration and might be slightly different for any particular future mission.

Based on the required accuracy of both the in-flight and the final attitude solution, the requirements not only on the sensor measurement accuracy itself, but also on the accuracy of the relevant calibration parameters and the data processing algorithms are defined. The data processing might be different for the in-flight and on-ground solution, it might also consider fusion of attitude data from multiple sensors such as SCA and ACC or IMU, or even the data from the steering mirror control loop. The optimal data processing strategy needs to be tested based on the measurement performance of all relevant attitude sensors. At the same time, the resulting optimal data processing strategy puts further requirements on the SCA measurement accuracy.

The required SCA measurement accuracy can be achieved by developing a proper sensor concept. On the one hand, such concept includes sensor design and performance, which is provided by the manufacturer. It is the task of the manufacturer to meet the requirements on the sensor measurement accuracy under consideration of the current available technology and the influence of satellite’s inner and outer environment of the sensor performance. On the other hand, the sensor concept also includes the sensor constellation onboard the satellite, i.e.

the number of SCA heads and their mounting geometry, and the algorithms for the raw data processing and for the possible combination of the attitude data from multiple SCA heads.

Furthermore, from the requirements on the in-flight and on-ground attitude accuracy, the needed accuracy of the calibration parameters related to the attitude determination sensors and their alignment (e.g. with respect to the ranging system, to the satellite body axes or to the other attitude sensors) is derived. Accordingly to these requirements, pre-flight and in-flight calibration maneuvers and data processing algorithms are designed.

In other words, based on this approach fundamental questions should be answered such as: what star camera measurement accuracy is needed; is such accuracy achievable with the current technology; what is the necessary mounting geometry of the star camera heads so that valid data from at least two cameras are available at any time; what is the necessary accuracy of the relevant calibration parameters, which calibration maneuvers are needed to be performed and how; is it necessary to combine the SCA data directly onboard the satellites and is any further fusion with attitude data from other sensors necessary for arriving at the required accuracy of the in-flight attitude solution; what is the required accuracy of the final attitude solution and does it allow the full exploitation of the measurement accuracy of the scientific observations; what are the limits of the designed attitude determination system; where are its weak points; what are the requirements on the satellite processing unit (hardware + software).

These and other questions need to be answered already in the mission design phase.

The overall approach for the design of the attitude determination system requires an iterative solution based on simulated observations, numerical simulations and testing of various scenarios, while considering the performance of all other satellite systems such as attitude control system, scientific instruments systems. Also, the end-to-end simulations are a necessary part of this iterative solution. This is a very complex system which is unique for any particular mission.

The numerical analysis is therefore beyond the scope is this thesis.

INSTRUMENT CONCEPT - Laser ranging instrument (LRI) - Accelerometer (ACC) - GNSS positioning (GNSS)

The role of attitude determination for satellite operation - Operational modes/ Pointing modes - Fuel consumption

The role of attitude determination for scientific data processing - post-processing of LRI, ACC, GNSS Requirements on the final attitude solution accuracy Requirements on accuracy of calibration parameters Design and concept of the calibration maneuvers

Requirements on SCA measurement accuracy SCA sensor concept - Sensor design & performance - SCA constellation (multiple heads) - mounting geometry - processing algorithms

Requirements on attitudedata fusion - SCA/ACC/IMU/SM - in-flight/on-ground - processing algorithms

SCIENCE AND MISSION REQUIREMENTS Requirements on the in-flight attitude solution accuracy

ATTITUDE CONTROL - Magnetic torquers - Cold gas thrusters OBSERVATIONS SIMULATIONS / NUMERICAL SIMULATIONS

Figure 7.5: Proposed approach for defining the requirements on the attitude determination system for the future missions based on their science and mission requirements. The aim is the define the requirements on sensor measurement accuracy, on the accuracy of the relevant calibration parameters and on the attitude data

processing

so does every moment carry the taste of eternity.

Sri Nisargadatta Maharaj

-Conclusions 8

We have presented the first comprehensive study on the role of attitude determination for inter-satellite ranging. This study is based on the data analysis from GRACE, which is the first and so far the only satellite mission using the inter-satellite ranging for the Earth’s gravity field observation. The inter-satellite ranging is a very challenging measurement technique especially because of its requirements on the attitude determination and control. It requires the two satellites being precisely pointed with their ranging antennas towards each other while keeping the maximum deviation of the antenna phase center from the LOS below∼5 mrad. The precise attitude determination is fundamental not only for the mission operation but also for the post-processing of the GRACE scientific observations needed for the gravity field recovery, i.e.

of the inter-satellite ranging observations, GPS observations and linear accelerations sensed by the accelerometer.

Today efforts are still ongoing to improve the accuracy of the GRACE gravity field models as the predicted accuracy has not been reached yet. On the one hand, this requires an improvement of the background models for the atmosphere, ocean and tides in order to reduce the impact of the aliasing effects coming from the improper spatial and temporal sampling of the satellite observations. On the other hand and most importantly, an improvement of the satellite observations is needed, because any uncorrected error directly propagates into the gravity field models. As the attitude data are necessary for the post-processing of all primary scientific observations, the analysis of the performance of the star cameras, which are the primary attitude determination sensors, and of the star camera data processing was absolutely essential in order to ensure their highest possible accuracy of the attitude data.

One of the current highest priorities of the geoscientific community is the continuation of the Earth’s gravity field observation from space. Therefore NASA and DLR decided to keep GRACE operating as long as possible and to launch a new satellite for the gravity field observation, the GRACE Follow-On, as soon as possible. Additionally, the next generation of the gravity field satellite missions is currently in the concept development phase. For both GRACE Follow-On and the future missions, the inter-satellite ranging was selected again to be the primary measurement technique. Profound understanding of the GRACE sensor characteristics and performance as well as the processing algorithms presented in this thesis is fundamental for the development of the future technology and for the optimal operation of the future missions. At the same time, it also allows for improvement of the current GRACE operation.

In the following, major findings of our data analysis are summarized and the implication of our research work is discussed. An outlook on further research work is provided as well.